1. Field of the Invention
The invention relates to a neutron-optical component array for the specific spectral shaping of neutron beams or pulses in a neutron guide or beam hole between a fast neutron source with several moderators of different structures arranged closely adjacent each other for generating slow neutrons of different energy spectra as well as for their radiation in predetermined radiation directions and to at least one place of experiment.
2. The Prior Art
Neutron beams serve in a broad spectrum of scientific examinations ranging from pure basic science to application-related examinations in the field of research of the structure of matter. Here, neutrons function quasi as sensors which penetrate into the matter. Neutrons impinging upon atoms of structured matter are either scattered in a manner characteristic of the atoms or they are absorbed by the atoms by emitting characteristic radiation. For most applications, as for instance in neutron scattering, it is necessary to provide slow neutrons which are generated by deceleration of fast neutrons obtained from nuclear reactions. Intensive neutron radiation of fast neutrons is primarily generated in research reactors either by splitting enriched uranium in a temporally constant flow or as pulses in spallation sources by crushing heavy atoms.
The specific deceleration of fast neutrons is primarily carried out by so-called “moderators” which are brought into contact with the fast neutron radiation. Stated in simple terms, these are collections of matter of gaseous, liquid or solid appearance which, at a predetermined temperature, have specific characteristics. By the interaction of fat neutrons with the preferably light atoms of the moderator matter, the high energetic neutrons are strongly decelerated to the point where their energies and wavelengths are of the requisite values for experiments with condensed matter. A neutron gas of kinetic energy distribution is produced which at a given temperature may be approximated by a Maxwellian velocity distribution. This is a theoretically derived function which assigns their relative abundance to the velocities of the atoms of a gas. The effective temperature of the Maxwellian spectrum of the neutron gas is somewhat higher, however, than the temperature of the moderator matter. In this connection it is to be mentioned that neutron reflectors such as, for instance, (heavy) water, lead, beryllium, graphite, etc. also generate slow neutrons, but with a spectrum different from the spectrum which may be approximated by the Maxwell spectrum. Nevertheless, reflectors which serve primarily to increase the flow of neutrons also contribute to neutron-deceleration, so that, in a broader sense, they may, as neutron-optical components, be grouped with the moderators. Premoderators such as water and all other structures of a neutron sources capable of emitting slow neutrons may also be counted among the group of moderators.
Depending upon the temperature of the moderator material, slow neutrons are differentiated between “hot”, “thermal”, and “cold” neutrons, so that the moderators may also be distinguished as “hot”, “thermal”, and “cold” moderators. In the present context, slow neutrons are those of a kinetic energy in the range of 1 eV and less. The energy of hot neutrons of higher velocity and lesser wavelength is in a range above 100 meV and are particularly suitable for scatter experiments with liquids. Thermal neutrons are of a kinetic energy in the range of between 10 meV and 100 meV, and the kinetic energy of cold neutrons lies in the range between 0.1 meV and 10 meV. Cold neutrons of relatively low velocity and large wavelength are above all of importance for applications of neutron scattering for examining biological substances. Depending on the kind of their primarily generated slow neutrons, a distinction is made between hot, thermal and cold moderators. A survey of possible moderator structures in a spallation source may be derived from paper I “Particle Transport Simulations of the Neutron Performance of Moderators of the ESS Mercury Target-Moderator-Reflection System” (downloadable from the Internet at http://www.hmi.de/bereiche/SF/ess/ESS_moderators3.pdf, state 18 January 2002). Examples thereof are the liquid hydrogen moderator with an operating temperature in the range of 25° K for generating cold neutrons and the water moderator using the ambient temperature as its operating temperature for generating thermal neutrons. However, a cold moderator also generates thermal and hot neutrons as well, and a thermal moderator also generates cold and hot neutrons, but always at a flow lower by an order of magnitude than the moderator which serves for generating primarily cold, thermal or hot neutrons.
To provide the correct required neutron spectrum for different experiments with slow neutrons, the known neutron sources operate with a combination of different moderators. From Paper II “The Spallation Neutron Source Project” by Jose R. Alsonso; Proceedings of the 1999 Particle Accelerator Conference, New York, 1999, pp. 574–578, (downloadable from the Internet at http://accelconf.web.cem.ch/accelconf/p99/PAPERS/FRAL1.pdf—(State 18. January 2002), it is known to position two water moderators tempered by room temperature below the level with the target material to be crushed and two super-critical hydrogen moderators with an operating temperature of 20° K above the target plane. Each moderator exclusively provides one or more of eighteen places of experiment with the slow neutron spectrum generated by it (see FIG. 9 and Chapter 6 of Paper II). A similar structure is also known from Paper III “5.3—Material Issues for Spallation Target by GeV Proton Irradiation” by W. Watanabe (downloadable from the Internet at http://www.ndc.tokai.jaeri.go.jp/nds/proceedings/1998/watanabe_n.pdf; state 18 January 2002). It describes a target-moderator-configuration for executing high intensity and high resolution experiments with cold neutrons, in which a coupled cold moderator with a premodulator and two thermal moderators are arranged closely adjacent the target in the region of the highest and fastest neutron radiation (see Paper III, Chapter 4 (2) to (4) and FIG. 2). As an important point, the paper refers to the close proximity notwithstanding, cross-talk between the individual moderators which effects the neutron intensity, can be prevented (see Paper III, Chapter 4 (ii)). For that reason, the moderators are arranged relative to each other at such angles that their forward and rearward radiation directions or emitted neutron beams are oriented in different spatial directions without overlapping each other. In this manner, each moderator supplies about four to eight places of experiment with a neutron beam of characteristic spectrum. Moreover, reflectors are arranged between the to levels for separating the spectra.
Proceeding from the known state of the art relating to the known application of moderators as described, for instance, in previously cited Paper III, it can be recognized that the provision of a neutron spectrum of slow neutrons required for a specific experiment as well as the generation thereof causes significant problems. In particular, with regard to the very complex and expensive structures of the neutron-optical components as well as the high protective measures which they require, the state of the art knows of no neutron spectrum for a single place of experiment. Each place is supplied with a neutron spectrum the maximum of which indicates the principally generated slow neutrons, from a directly associated moderator type. Changes in the spectrum of the neutron beam at a place of experiment may be realized only by significant structural changes in the structure of the moderator at extended down-times of the neutron source. Experiments in energy ranges broader than the one of a single slow neutron form are not possible or they are very inefficient.